1. Field of the Invention
The invention relates to a gas detector provided with a porous gas sensitive element.
2. Description of the Prior Art
The electrical resistivity of a sintered body of metal oxide semiconductors such as titanium dioxide varies with the type of gas it contacts therefore making such materials useful as gas sensitive elements as shown in U.S. Pat. Nos. 3,886,785 and 3,933,028. Such elements are typically supported with a refractory adhesive in a slot in the upper end of the ceramic insulator tube so as to provide a gas detector. However, a conventional gas detector, which is generally shown at 1 in FIG. 1 is provided with electrodes 7 that extend from the top to the bottom of the element 1 and which extend halfway down into a porcelain tube 4. Therefore, a refractory adhesive 9 impregnates into the porous lower half of the element 1, and gas sensitivity of sensor element is diminished, and the response time of the element is undesirably increased.
A gas sensitive element for use in a detector for the air/fuel ratio of vehicle exhaust gas must be such that it exhibits a sudden change in its electrical resistance at the theoretical mixing ratio (.lambda.=1). The electrical resistance of such an element also varies with temperature. To detect the theoretical mixing ratio, it must have a certain amount of overlap for sudden changes in electrical resistance (the overlap to be hereunder referred to as .DELTA.W.multidot.B) at the theoretical mixing ratio within the operating temperature range of the air/fuel ratio detector. That is, it is advantageous to provide wide overlapping range relative to different temperature in order to detect the gas irrespective of its temperature. Such overlap is shown in FIG. 5. A conventional gas sensitive element however does not have a sufficient level of .DELTA.W.multidot.B to provide for accurate detection of the theoretical mixing ratio.